Darwin’s original letter to Thomas Thornton, Esq., saying I do not have proof ... (1861)
Darwin’s conception of the theory of natural selection(After an unpublished letter of 1861)
The letter was presented to the British Museum in 1908 by the widow of THOMAS THORNTON, Esq. It covers eight small pages (18 1/2 × 12 cm.) in DARWIN’S own hand and bears the registration number Add. MS. 37725, f. 6.